[llvm-dev] new pass manager version of opt vs. legacy pass manager version

[Madhur Amilkanthwar via llvm-dev]

On Thu, Apr 22, 2021 at 9:03 PM Snider, Todd via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello All,
>
>
>
> My development group has been maintaining a downstream version of the
> monorepo that stays in sync with the upstream “main” branch, but we are
> still using the legacy pass manager in our local copy of the monorepo.
>
>
>
> We’ve recently encountered a few instances of lit tests that are failing
> when run with the legacy pass manager version of opt, but pass when run
> with the new pass manager version of opt.
>
I am curious, why would the switch to the new pass manager make them fail
as those tests must have been well running with the old pass manager. Are
these tests present in the trunk?

The situation raises a couple of questions:
>
>    - Is the legacy pass manager behavior being adequately tested by the
>    buildbots?
>    - What expectation should there be that legacy pass manager behavior
>    will be maintained in light of changes made to code that affects both the
>    new pass manager version and the legacy pass manager version of opt?
>
>
>
> I suspect that my group is not the only ones trying to stay in sync with
> the upstream LLVM main branch and keep using the legacy pass manager, and I
> anticipate the only long-term remedy for our situation is to move to using
> the new pass manager as soon as we can.
>
>
>
> Thoughts?
>
IMHO, switch to the new pass manager may not happen overnight for many
orgs. I believe the new pass manager has a different pass ordering and
invalidation mechanism as compared to the old one so there could be impact
on compile-time and application runtime performance.


> Regards.
>
>
>
> Todd Snider
>
> Texas Instruments Incorporated
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-- 
*Disclaimer: Views, concerns, thoughts, questions, ideas expressed in this
mail are of my own and my employer has no take in it. *
Thank You.
Madhur D. Amilkanthwar
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